A PHOTOMETRIC AND RADIAL-VELOCITY ANALYSIS OF THE INTERMEDIATE-AGE OPEN CLUSTER NGC-752

Using all available proper-motion and radial-velocity data, including new radial-velocity observations obtained for this investigation, probable members of the open cluster NGC 752 have been identified. Photoelectric data on six systems have been transformed and collated to form an internally consistent sample on the BV system. Binaries have been identified using photometric and radial-velocity data, including a photographic survey for variability and the radial-velocity observations of this study. Analysis of the data leads to the following cluster parameters and their probable errors: E(B - V) = 0.035 +/- 0.005 mag, [Fe/H] = -0.15 +/- 0.05 dex, and (m - M) = 8.25 +/- 0.10 mag. The spread in color among stars in the color-magnitude diagram (cmd) along the main sequence from the turnoff to the unevolved main sequence is the consequence of a rich population of binaries. Due to its age and the comprehensive data available for the cluster, NGC 752 provides an ideal test of a variety of evolutionary phenomena. Comparison with theoretical isochrones normalized in an internally consistent manner leads to the conclusion that the morphology and distribution of stars in the cmd can best be matched using models that include convective overshoot, particularly those of Schaller et al. (1992, A&AS, 96, 269). Despite their differences, the traditional and the overshoot isochrones both imply very similar ages, 1.9 +/- 0.2 Gyr and 1.7 +/- 0.1 Gyr, respectively, for the cluster. The Li abundances for the giants confirm that the giant branch is dominated by clump stars and first-ascent giants below the luminosity of the clump. The position and size of the Li dip among the main-sequence stars, compared to the Hyades, is readily explained by stellar evolution with convective overshoot. It is predicted that among turnoff stars in the intermediate-age range Li will cease to be a unique function of age at a given color. Chromospheric flux is shown to be a well-defined function of color for single, unevolved stars, identical to that found for the Hyades, and the relation for NGC 752 falls within the Vaughan-Preston gap. However, the slope of the relation requires that increasing color implies increasing age for the bluer portion of the weak-emission boundary. The combined effect of small samples, random errors, emission limits, a possible selection bias in favor of turnoff stars, and metallicity corrections is to render highly questionable any interpretation of time-variable star formation within the Galaxy based upon chromospheric ages.